Break 2 oz. chocolate into small pieces; place in same bowl used to prepare cake batter. Add boiling water; let stand 1 min. Stir until chocolate is completely melted. Pour over batter in 4 qt. slow cooker; cover with lid.

Cook on LOW 2-1/4 to 2-3/4 hours (or HIGH 1-1/2 to 1-3/4 hours) or until toothpick inserted near center comes out with fudgy crumbs. Turn slow cooker off.

Microwave remaining chocolate and COOL WHIP in microwaveable bowl on HIGH 1 to 1-1/2 min. or until chocolate is completely melted and mixture is well blended, stirring with whisk after 45 sec. Spread over cake; sprinkle with nuts. Serve warm.